This invention relates to a low-pressure discharge lamp comprising a double spiral shaped discharge tube which includes two spiral shaped tube portions. The lamp is provided with a cold chamber portion connecting the ends of the spiral shaped tube portions.
Low pressure discharge lamps are well known in the art. These lamps contain small doses of mercury which radiates under the influence of the discharge arc. In order to achieve maximum light output, it is required that the mercury vapour is adjusted and stabilized on a well-defined partial pressure. This is possible by forming a so-called cold chamber on the discharge tube, and by selecting the appropriate temperature in the cold chamber which is the coldest point of the gas discharge tube.
A spiral-shaped compact fluorescent lamp is disclosed in the Patent No. DD 212 843 published in the former German Democratic Republic. This lamp comprises a straight gas discharge tube portion surrounded by another spiral-shaped gas discharge tube portion with one thread. This known lamp has not prevailed in the practice, since the manufacturing of the two different gas discharge tube portions with different shapes requires two separate production lines which increases production cost. Also, the overall visual appearance of the lamp and its light distributions is not completely satisfactory.
German Patent Application No. DE 41 33 077 discloses another spiral shaped discharge lamp, but with a double spiral shaped discharge tube. In this known discharge lamp, the cold chamber is positioned at the top of the lamp, between the two ends of the tube portions constituting the strands of the double spiral. The cold chamber is formed by an annular widening of the discharge tube. However, the light distribution of the lamp in the region of the cold chamber still needs improvement because a relative large portion of the enveloping surface is not utilized as lighting surface, particularly in the direction along the axis of the lamp, towards the end which is further away from the lamp housing. This is of particular importance when the lamp is screwed in a socket on the ceiling, with the cold chamber facing downwards.
Therefore, there is a need for a discharge lamp which exhibits improved light distribution also at the top of the lamp, and which has an efficient cold chamber for optimal performance of the lamp, while the possibility of an economic manufacturing of the discharge tube is also maintained.
In an embodiment of the present invention, there is provided a low-pressure discharge lamp comprising a double spiral shaped discharge tube. The discharge tube includes two spiral shaped tube portions. The tube portions define a central axis of the discharge tube, in the sense that each of the tube portions are wound around a theoretical axis, and the two axis substantially coincide. A cold chamber portion connects the ends of the spiral shaped tube portions. The cold chamber portion has a first transversal dimension, the first transversal dimension being defined as a transversal dimension measured substantially perpendicular to the central axis. This first transversal dimension of the cold chamber portion is larger than the diameter of the tube portions. The cold chamber portion has a second transversal dimension. This second transversal dimension is measured substantially parallel to the central axis. The second transversal dimension of the cold chamber portion substantially corresponds to the diameter of the tube portions.
The lamp with the cold chamber portion of the above described design has an improved luminance distribution combined with enhanced mechanical stability, as compared with known cold chamber designs. The cold chamber portion and the two tube portions of the discharge tube may be easily formed starting from a single integral glass tube, thereby avoiding imperfect joints between discharge tube sections.